There have been suggestions that the Portuguese winger could be on his way out of Old Trafford after failing to command a regular starting place this season.
Arsenal have been linked with the 26-year-old, who is currently sidelined with a hamstring problem, but Ferguson has dashed their hopes.
"We won't be letting him go. We need a Nani," said the Scot in the Sunday Mirror.
"His contract isn't up for a year and a half. He offers something different from the other players. He's an incredible talent, the boy.
"Unfortunately, he's injured at the moment. We have sent him over to Dubai for a break, to help him in his recovery.
"He's not far away but he's far enough away. I don't think I will get him until the middle of January.
"He's got a future here. Why would I want to let him go?"
Ferguson has also confirmed that he is unlikely to make any moves in January.
"To get a player in January is very difficult because it has to be a player who is going to improve on what you have already got," he said.
"Big clubs don't want to sell their best players and I don't see someone like Chelsea buying a player who is not available for Europe.
"I think there will probably be one big signing in January - whether that will be Arsenal or Chelsea, I don't know.
"I can't see Atletico Madrid selling Falcao, but David Villa is available at Barcelona because he isn't getting a game. But we're in good shape. Once we get all our players fit, we'll have a squad of 25 or 26."
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